U.S. patent application number 11/742154 was filed with the patent office on 2007-11-22 for estimation and control of head fly height.
This patent application is currently assigned to Seagate Technology LLC. Invention is credited to Jim Fitzpatrick, Barry Henry, Jihao Luo, Jesse Speckhard, Baoliang Zhang.
Application Number | 20070268612 11/742154 |
Document ID | / |
Family ID | 38711742 |
Filed Date | 2007-11-22 |
United States Patent
Application |
20070268612 |
Kind Code |
A1 |
Fitzpatrick; Jim ; et
al. |
November 22, 2007 |
ESTIMATION AND CONTROL OF HEAD FLY HEIGHT
Abstract
Various embodiments are disclosed that control head fly height
based on estimates of head fly height. The fly height clearance
between the head and a data storage media is estimated. Heating of
the head by a heater element is then regulated in response to the
estimated fly height.
Inventors: |
Fitzpatrick; Jim; (Sudbury,
MA) ; Luo; Jihao; (Shrewsbury, MA) ; Zhang;
Baoliang; (El Toro, CA) ; Speckhard; Jesse;
(Douglas, MA) ; Henry; Barry; (Shrewsbury,
MA) |
Correspondence
Address: |
Myers Bigel Sibley & Sajovec, P.A.
P.O. Box 37428
Raleigh
NC
27627
US
|
Assignee: |
Seagate Technology LLC
|
Family ID: |
38711742 |
Appl. No.: |
11/742154 |
Filed: |
April 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60747598 |
May 18, 2006 |
|
|
|
60747636 |
May 18, 2006 |
|
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Current U.S.
Class: |
360/75 ; 360/31;
360/60; G9B/5.026; G9B/5.231 |
Current CPC
Class: |
G11B 5/02 20130101; G11B
5/6005 20130101; G11B 5/6064 20130101; G11B 5/607 20130101; G11B
5/3136 20130101; G11B 2005/0005 20130101 |
Class at
Publication: |
360/75 ; 360/60;
360/31 |
International
Class: |
G11B 21/02 20060101
G11B021/02; G11B 27/36 20060101 G11B027/36; G11B 15/04 20060101
G11B015/04 |
Claims
1. A method of controlling head fly height comprising: estimating
fly height clearance between a head and a data storage media in
response to heater signal levels applied to a heater element to
heat the head; and regulating heating of the head by the heater
element in response to the estimated fly height.
2. The method of claim 1, further comprising: generating a
repository of calibrated fly height values for a range of heater
signal levels applied to the heater element to heat the head,
wherein the fly height is estimated in response to the repository
of calibrated fly height values and a present level of the heater
signal applied to the heater element.
3. The method of claim 2, further comprising: generating the
repository of calibrated fly height values further for a range of
ambient air temperatures, wherein the fly height is estimated in
response to the repository of calibrated fly height values, a
present level of the heater signal applied to the heater element,
and a present air temperature.
4. The method of claim 2, wherein the media comprises a disk, and
further comprising: generating the repository of calibrated fly
height values for the range of heater signal levels applied to the
heater element to heat the head and at a plurality of radial
locations across the disk, wherein the fly height is estimated in
response to the repository of calibrated fly height values, a
present level of the heater signal applied to the heater element,
and a radial location on the disk where data is to be written/read
by the head.
5. The method of claim 1, further comprising: generating a
repository of calibrated fly height values for a plurality of heads
relative to a plurality of data storage media for a range of heater
signal levels applied to heater elements to heat the heads, wherein
the fly height is estimated in response to the repository of
calibrated fly height values and in response to which of the
plurality of heads is selected to write/read data and a present
level of the heater signal applied to the heater element to heat
the selected head.
6. The method of claim 1, wherein regulating heating of the head
comprises: regulating a heater signal to increase heating by the
heater element in response to the estimated fly height exceeding an
upper target threshold; and regulating the heater signal to
decrease heating by the heater element in response to the estimated
fly height being less than a lower target threshold.
7. The method of claim 6, further comprising: inhibiting writing
and/or reading through the head in response to the estimated fly
height being less than a lower operational threshold; and
inhibiting writing through the head in response to the estimated
fly height being greater than an upper operational threshold.
8. The method of claim 7, wherein writing is inhibited in response
to a first lower operational threshold and reading is inhibited in
response to a second lower operational threshold that is different
than the first lower operational threshold.
9. The method of claim 1, wherein regulating heating of the head
comprises, in response to selection of a head for reading/writing
on the media, regulating a heater signal to increase heating by the
heater element until the estimated fly height is less than an upper
target threshold, and decreasing heating by the heater element in
response to the estimated fly height becoming less than the upper
target threshold
10. The method of claim 9, further comprising, enabling writing of
data through the head onto the media in response to the estimated
fly height becoming less than a write gate upper limit.
11. The method of claim 1, wherein the fly height is estimated in
response to length of a data segment written through the head to
the media.
12. The method of claim 11, wherein estimating fly height
comprises: measuring duty cycle of writing on the media; deceasing
the fly height estimate in response to increased duty cycle; and
increasing the fly height estimate in response to decreased duty
cycle.
13. The method of claim 1, wherein the fly height is estimated in
response to length of a data segment that is about to be written
through the head.
14. The method of claim 1, wherein the fly height is estimated
based on a combination of an estimate of head fly height at a
sensed air temperature, an estimate of head pole-tip-protrusion
from head heating from writing data, and an estimate of head
pole-tip-protrusion from head heating by the heater element.
15. The method of claim 14, wherein: the estimate of head
pole-tip-protrusion from head heating from writing data is based on
the following equation:
PTP.sub.W(k)=A.sub.1(.alpha..sub.1(1-e.sup.(-kT/.tau..sup.1.sup.))+(1-.al-
pha..sub.1)(1-e.sup.(-kT/.tau..sup.2.sup.))), where PTP.sub.W(k)
represents the head pole-tip-protrusion at time k, T represents a
servo spoke location of the head, A.sub.1 represents amplitude of
pole-tip-protrusion over an operable range of writing-induced
heating, .alpha..sub.1 represents a gain factor, and .tau..sub.1
and .tau..sub.2 are time constants indicative of the rate of change
of pole-tip-protrusion in response to writing data through the
head; and the estimate of head pole-tip-protrusion from head
heating by the heater element is based on the following equation:
PTP.sub.H(k)=A.sub.2(.alpha..sub.2(1-e.sup.(-kT/.tau..sup.3.sup.))+(1-.al-
pha..sub.2)(1-e.sup.(-kT/.tau..sup.4.sup.))), where PTP.sub.H(k)
represents the head pole-tip-protrusion at time k, T represents a
servo spoke location of the head, A.sub.2 represents amplitude of
pole-tip-protrusion over an operable range of heating by the heater
element, .alpha..sub.2 represents a gain factor, and .tau..sub.3
and .tau..sub.4 are time constants indicative of rate of change of
pole-tip-protrusion in response to a heater signal applied to the
heater element.
16. A circuit comprising: a fly height controller that estimates
fly height clearance between a head and a data storage media in
response to heater signal levels applied to a heater element to
heat the head, and regulates heating of the head by the heater
element in response to the estimated fly height.
17. The circuit of claim 16, wherein the fly height controller
estimates fly height based on a combination of an estimate of head
fly height at a sensed air temperature, an estimate of head
pole-tip-protrusion from head heating from writing data, and an
estimate of head pole-tip-protrusion from head heating by the
heater element.
18. A method of controlling head fly height comprising: estimating
fly height clearance between a head and a data storage media in
response to heater signal levels applied to a heater element to
heat the head and in response to length of a data segment written
through the head; and regulating heating of the head by the heater
element in response to the estimated fly height.
19. The method of claim 18, further comprising estimating the fly
height further in response to air temperature.
20. The method of claim 18, further comprising: measuring duty
cycle of writing on the media; decreasing the fly height estimate
in response to increased duty cycle; and increasing the fly height
estimate in response to decreased duty cycle.
Description
RELATED APPLICATION
[0001] This application claims the benefit of and priority to U. S.
Provisional Patent Application No. 60/747,598, filed May 18, 2006,
and to U.S. Provisional Patent Application No. 60/747,636, filed
May 18, 2006, the disclosures of which are hereby incorporated
herein by reference as if set forth in their entirety.
FIELD
[0002] The present invention generally relates to sensor clearance
control and, more particularly, to controlling fly height of a
read/write head in a data storage device.
BACKGROUND
[0003] Data storage devices, such as disk drives, allow host
computers to store and retrieve large amounts of digital data in a
fast and efficient manner. A typical disk drive includes a
plurality of magnetic recording disks which are mounted to a
rotatable hub of a spindle motor and rotated at a high speed. An
array of read/write heads is disposed adjacent to data storage
surfaces of the disks to transfer data between the disks and a host
computer. The heads can be radially positioned over the disks by a
rotary actuator and a closed loop servo system, and can fly in
close proximity to the surfaces of the disks upon air bearings. The
heads each typically contain a separate read element and write
element.
[0004] Higher data storage density on the disks may be obtained by
reading and writing data on narrower tracks on the disks and by
maintaining a corresponding smaller fly height gap between the
heads and the data storage surfaces. The fly height of a head can
vary in response to air density changes in the disk drive, and in
response to head temperature variations, such as while writing,
which can affect the distance that the tip of the head protrudes
therefrom (i.e., pole-tip protrusion). Some disk drives use a
heater to controllably heat the head in order to vary the fly
height of the head.
[0005] Maintaining the head fly height within an acceptable range
is becoming increasingly more difficult as that range is reduced to
obtain higher data storage densities. Operation outside the
acceptable range may result in an unacceptable read/write bit error
rate and/or undesirable contact between a head and a data storage
surface and potential loss of data and/or damage to the data
storage surface.
SUMMARY
[0006] Various embodiments are disclosed that control head fly
height based on estimates of head fly height. The fly height
clearance between the head and a data storage media is estimated in
response to heater signal levels applied to a heater element to
heat the head. Heating of the head by the heater element is then
regulated in response to the estimated fly height.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a perspective view of a disk drive with electronic
circuits that are configured in accordance with some
embodiments.
[0008] FIG. 2 is a block diagram of an exemplary head disk assembly
of the disk drive.
[0009] FIG. 3 is a block diagram of a portion of the controller of
the disk drive shown in FIG. 1 and associated methods that are
configured in accordance with some embodiments.
[0010] FIG. 4 is more detailed block diagram of the fly height
controller of FIG. 3 and associated methods that are configured in
accordance with some embodiments.
[0011] FIG. 5 shows a graph that illustrates methods and operation
of the fly height controller of FIGS. 3 and 4 for regulating heater
element power to attempt to obtain and maintain head fly height
clearance within an acceptable range.
[0012] FIG. 6 shows two graphs that illustrate further methods and
operation of the fly height controller of FIGS. 3 and 4 for
regulating heater element power to attempt to obtain and maintain
head fly height clearance within an acceptable range without data
writing events.
[0013] FIG. 7 shows two graphs that illustrate further methods and
operation of the fly height controller of FIGS. 3 and 4 for
regulating heater element power to attempt to obtain and maintain
head fly height clearance within an acceptable range with data
writing events.
DETAILED DESCRIPTION
[0014] Specific exemplary embodiments of the invention now will be
described with reference to the accompanying drawings. This
invention may, however, be embodied in many different forms and
should not be construed as limited to the embodiments set forth
herein; rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will convey the scope
of the invention to those skilled in the art. The terminology used
in the detailed description of the particular exemplary embodiments
illustrated in the accompanying drawings is not intended to be
limiting of the invention.
[0015] It will be understood that, as used herein, the term
"comprising" or "comprises" is open-ended, and includes one or more
stated elements, steps and/or functions without precluding one or
more unstated elements, steps and/or functions. As used herein, the
singular forms "a", "an" and "the" are intended to include the
plural forms as well, unless the context clearly indicates
otherwise. As used herein the terms "and/or" and "/" include any
and all combinations of one or more of the associated listed items.
It will be understood that, although the terms first, second, etc.
may be used herein to describe various steps, elements and/or
regions, these steps, elements and/or regions should not be limited
by these terms. These terms are only used to distinguish one
step/element/region from another step/element/region. Thus, a first
step/element/region discussed below could be termed a second
step/element/region without departing from the teachings. Like
numbers refer to like elements throughout the description of the
figures.
[0016] The present invention may be embodied in hardware and/or in
software (including firmware, resident software, micro-code, etc.).
Consequently, as used herein, the term "signal" may take the form
of a continuous waveform and/or discrete value(s), such as digital
value(s) in a memory or register.
[0017] The present invention is described below with reference to
block diagrams of disk drives, disks, controllers, and operations
according to various embodiments. It is to be understood that the
functions/acts noted in the blocks may occur out of the order noted
in the operational illustrations. For example, two blocks shown in
succession may in fact be executed substantially concurrently or
the blocks may sometimes be executed in the reverse order,
depending upon the functionality/acts involved. Although some of
the diagrams include arrows on communication paths to show what may
be a primary direction of communication, it is to be understood
that communication may occur in the opposite direction to the
depicted arrows.
[0018] A simplified diagrammatic representation of a disk drive,
generally designated as 10, is illustrated in FIG. 1. The disk
drive 10 includes a disk stack 12 (illustrated as a single disk in
FIG. 1) that is rotated about a hub 14 by a spindle motor 15 (FIG.
2). The spindle motor 15 is mounted to a base plate 16. An actuator
arm assembly 18 is also mounted to the base plate 16. The disk
drive 10 is configured to store and retrieve data responsive to
write and read commands from a host device. A host device can
include, but is not limited to, a desktop computer, a laptop
computer, a personal digital assistant (PDA), a digital video
recorder/player, a digital music recorder/player, and/or another
electronic device that can be communicatively coupled to store
and/or retrieve data in the disk drive 10.
[0019] The actuator arm assembly 18 includes a read/write head 20
(or transducer) mounted to a flexure arm 22 which is attached to an
actuator arm 24 that can rotate about a pivot bearing assembly 26.
The read/write head, or simply head, 20 may, for example, include a
magnetoresistive (MR) element and/or a thin film inductive (TFI)
element. The actuator arm assembly 18 also includes a voice coil
motor (VCM) 28 which radially moves the head 20 across the disk
stack 12. The spindle motor 15 and actuator arm assembly 18 are
coupled to a controller, read/write channel circuits, and other
associated electronic circuits 30 which are configured in
accordance with at least one embodiment, and which can be enclosed
within one or more integrated circuit packages mounted to a printed
circuit board (PCB) 32. The controller, read/write channel
circuits, and other associated electronic circuits 30 are referred
to below as a "controller" for brevity. The controller 30 may
include analog circuitry and/or digital circuitry, such as a gate
array and/or microprocessor-based instruction processing
device.
[0020] Referring now to the illustration of FIG. 2, the disk stack
12 typically includes a plurality of disks 34, each of which may
have a pair of disk surfaces 36. The disks 34 are mounted on a
cylindrical shaft and are rotated about an axis by the spindle
motor 15.
[0021] The actuator arm assembly 18 includes a plurality of the
heads 20, each of which is positioned to be adjacent to a different
one of the disk surfaces 36. Each head 20 is mounted to a
corresponding one of the flexure arms 22. The VCM 28 operates to
move the actuator arm 24, and thus moves the heads 20 across their
respective disk surfaces 36. The heads 20 are configured to fly on
an air cushion relative to the data recording surfaces 36 of the
rotating disks 34 while writing data to the data recording surface
responsive to a write command from a host device or while reading
data from the data recording surface to generate a read signal
responsive to a read command from the host device.
[0022] FIG. 2 further illustrates tracks 40 and spokes 43 on the
disks 34. Data is stored on the disks 34 within a number of
concentric tracks 40 (or cylinders). Each track 40 is divided into
a plurality of radially extending sectors 42 separated by radially
extending spokes 43. Each sector is further divided into a servo
sector and a data sector. The servo sectors of the disks 34 are
used, among other things, to accurately position the head 20 so
that data can be properly written onto and read from a selected one
of the disks 34. The servo sectors may include a DC erase field, a
preamble field, a servo address mark field, a track number field, a
spoke number field, and a servo burst field (e.g.,
circumferentially staggered and radially offset A, B, C, D servo
bursts). The data sectors are where data received as part of a
host-initiated write command is stored, and where data can be read
in response to a host-initiated read command.
[0023] FIG. 3 is a block diagram of a host device 60 that is
communicatively connected to a portion of the controller 30 of the
disk drive 10 shown in FIG. 1 according to some embodiments. The
controller 30 can include a data controller 52, a servo controller
53, a read write channel 54, a buffer 55, a fly height controller
57, and an air temperature sensor 58. Although the controllers 52,
53, and 57, the buffer 55, and the read write channel 54 have been
shown as separate blocks for purposes of illustration and
discussion, it is to be understood that their functionality
described herein may be integrated within a common integrated
circuit package or distributed among more than one integrated
circuit package. The head disk assembly (HDA) 56 can include a
plurality of the disks 34a-b, a plurality of the heads 20a-d
mounted to the actuator arm assembly 18 and positioned adjacent to
different data storage surfaces of the disks 34a-b, the VCM 28, and
the spindle motor 15. In general, there may be two heads 20 per
disk 34. Thus, in a 4-disk platter drive, there may be eight heads
20.
[0024] Write commands and associated data from the host device 60
are buffered in the buffer 55. The data controller 52 is configured
to carry out buffered write commands by formatting the associated
data into blocks with the appropriate header information, and
transferring the formatted data from the buffer 55, via the
read/write channel 54, to logical block addresses (LBAs) on the
disk 34 identified by the associated write command.
[0025] The read write channel 54 can operate in a conventional
manner to convert data between the digital form used by the data
controller 52 and the analog form of a write current conducted
through a selected head 20 in the HDA 56. The read write channel 54
provides servo positional information read from the HDA 56 to the
servo controller 53. The servo positional information can be used
to detect the location of the head 20 in relation to LBAs on the
disk 34. The servo controller 53 can use LBAs from the data
controller 52 and the servo positional information to seek the head
20 to an addressed track and block on the disk 34, and to maintain
the head 20 aligned with the track while data is written/read on
the disk 34.
[0026] When a head 20 is selected for reading/writing, its fly
height is typically above an acceptable flight height range where
the head 20 should/must be located when reading/writing data on the
disk 34. Accordingly, in response to selection of a head 20, the
flight controller 57 heats the head 20 using a heater element to
lower the head fly height to within the acceptable range. Upon
reaching the acceptable range, reading/writing may be carried out
through the selected head 20.
[0027] In accordance with some embodiments, the fly height
controller 57 includes a fly height estimator 300 and a heater
controller 304. The heater controller 304 controls heating by a
heater element that heats the head 20. The fly height estimator 300
repetitively estimates the fly height of the head 20 relative to
the disk 34 as data is written/read therefrom. The system regulates
head heating by the heater controller 304 to drive the head 20 to
within an acceptable fly height range and then attempts to maintain
the head fly height within that range.
[0028] For example, in response to selection of the head 20 for
writing, the fly height estimator 300 repetitively estimates head
fly height as it regulates heating of the head to drive the fly
height to within a range that is acceptable for writing. The fly
height estimator 300 then continues to repetitively estimate head
fly height and the fly height controller 304 regulates head
heating, in response to the estimates, in an attempt to maintain
head fly height within the acceptable range while the head 20 is
writing data, which may include a sequence of writing groups of
data blocks with gaps therebetween.
[0029] The heater controller 304 controls head fly height by
regulating the power that is provided to a heater element that
heats a selected head. With reference to FIG. 3, the HDA 56 may
include a plurality of heater elements 68a-d attached, or otherwise
thermally connected, to different ones of the heads 20a-d. The
heater controller 304 generates heater signal 59 which is conducted
through the heater elements 68a-d to generate heat therefrom and,
thereby, heat the heads 20a-d. The heater controller 304 regulates
the height adjustment signal 59 to control heating of the heads
20a-d and cause a controlled amount of pole-tip-protrusion
(thermally-induced elastic deformation) from the heads 20a-d and,
thereby, control fly heights of the heads 20a-d.
[0030] Although one heater signal 59 has been shown in FIG. 3, and
which may be used to separately control heating by different ones
of the heater elements 68a-d, it is to be understood that more
heater signals 59 may be used to control the heater elements 68a-d
and that, for example, the heater elements 68a-d may be controlled
by individual heater signals 59.
[0031] As data is written, the head 20 is heated by the write
current and its temperature can continue to rise to higher levels
as the length of data (e.g., the number of blocks) that is written
substantially continuously on the disk 34 increases. As head
temperature increases, head fly height can decrease due to, for
example, increasing head pole-tip-protrusion. When gaps occur
between writes, the head 20 can cool, causing decreased
pole-tip-protrusion and corresponding increase in fly height.
Accordingly, the head 20 can be subjected to abrupt temperature
fluctuations, which, if left uncompensated, may result in abrupt
changes in fly height.
[0032] The fly height estimator 300 can estimate the fly height for
a selected head 20 (e.g., 20a or another one of the heads 20b-d) in
response to air temperature, which may be sensed by a temperature
sensor 58, and/or in response to air pressure, which may be sensed
by a pressure sensor. The fly height estimator 300 can also
estimate fly height of the selected head 20a in response to a level
of heating of the selected head 20a by the heater elements 68a,
length of a data segment last written through the head 20a, and/or
in response to write duty cycle (i.e., ratio of writing time to
total time) for writing a plurality of data segments (e.g., a
plurality of data blocks) through the head 20a with gaps
therebetween.
[0033] The fly height estimator 300 may also compensate for the
head heating that will occur in the future as planned writes are
carried out. For example, the fly height estimator 300 may
determine the length of one or more data segments in the buffer 55
that are about to be written on the disk 34, and may estimate the
effect of such writing on head fly height and adjust the heater
voltage in preparation for this change.
[0034] While data is being written/read, the fly height estimator
300 may generate a decreased fly height estimate in response to
increased air temperature, increased heating by a heater element,
and/or following writing of a longer data segment or increased
write duty cycle (i.e., increased write power dissipation in the
head 20 and associated increase in pole-tip-protrusion). Similarly,
the fly height estimator 300 may generate an increased fly height
estimate in response to decreased air temperature, decreased
heating by the heater element, and/or increased gap between
writes.
[0035] The fly height estimator 300 may periodically adjust
(calibrate) its estimates of head fly height in response to fly
height measurements carried out through the head 20a-d. The fly
height measurements may be carried out by comparing the magnitude
of a servo signal, which is generated by reading defined fields in
the servo sectors using a selected head, to a known relationship
between signal magnitude and head fly height.
[0036] FIG. 4 is more detailed block diagram of the fly height
controller 57 of FIG. 3 and associated methods that are configured
in accordance with some embodiments. Referring to FIG. 4, the fly
height estimator 300 can include a fly height table 310 of
parametric values that define for each of the heads 20a-d an
estimate of the head fly height when the head is subjected to a
defined air temperature and/or air pressure, subjected to a level
of heating from a corresponding one of the heater elements 68a-d,
and/or subjected to a level of heating from writing a data segment
having a defined length and/or writing at a certain duty cycle.
[0037] The parametric values of the fly height table 310 may be
defined during the design of the disk drive 10 and/or may be
calibrated during factory testing of the disk drive 10, the
operations by which are collectively identified as parametric
calibration 320. For example, the fly heights of the heads 20a-d
may be individually measured during factory testing while the heads
20a-d are subjected to a range of air temperatures, subjected to a
range of heating levels by the heater elements 68a-d, and/or
subjected to a range of write current durations and/or write duty
cycles. The fly height table 310 may be further calibrated during
operation of the disk drive 10 in response to fly height
measurements, such as described above by reading defined fields in
the servo sectors or elsewhere on the disks 34a-b.
[0038] The fly height estimate by the fly height estimator 300 may
be generated based on a combination of intermediate estimates, such
as an estimate of head fly height at a sensed air temperature
combined with an estimate of head pole-tip-protrusion from head
heating by the heater element 68 and an estimate of head
pole-tip-protrusion from writing data. The head fly height relative
to a disk 34 may be determined by subtracting the summed estimates
of head pole-tip-protrusion, due to heater element heating and head
writing, from the estimate of head fly height at a particular air
temperature (i.e., head pole-tip-protrusion decreases the head fly
height gap at a particular air temperature).
[0039] For example, pole-tip-protrusion distance due to head
heating while writing data may be repetitively estimated over a
time increment k based on the following Equation 1:
PTP.sub.W(k)=A.sub.1(.alpha..sub.1(1-e.sup.(kT/.tau..sup.1.sup.))+(1-.al-
pha.)(1-e.sup.(-kT/.tau..sup.2.sup.))), (Equation 1)
where PTP.sub.W(k) represents the writing-induced head
pole-tip-protrusion distance at time increment k, T represents a
servo spoke location of the head 20, A.sub.1 represents a
calibrated amplitude of pole-tip-protrusion over an operable range
of writing-induced heating, .alpha..sub.1 represents a gain factor,
and .tau..sub.1 and .tau..sub.2 are calibrated time constants that
are indicative of the rate of change of pole-tip-protrusion in
response to writing data through the head.
[0040] The pole-tip-protrusion distance due to head heating by the
heater element 68 may be repetitively estimated over the time
increment k based on the following Equation 2:
PTP.sub.H(k)=A.sub.2(.alpha..sub.2(1-e.sup.(-kT/.tau..sup.3.sup.))+(1-.a-
lpha..sub.2)(1-e.sup.(-kT/.tau..sup.4.sup.))), (Equation 2)
where PTP.sub.H(k) represents the heater-induced head
pole-tip-protrusion distance at time increment k, T represents a
servo spoke location of the head 20, A.sub.2 represents a
calibrated amplitude of pole-tip-protrusion over an operable range
of heating by the heater element, .alpha..sub.2 represents a gain
factor, and .tau..sub.3 and .tau..sub.4 are calibrated time
constants that are indicative of the rate of change of
pole-tip-protrusion in response to a heater signal applied to the
heater element.
[0041] The time increment k at which the fly height estimates are
updated may or may not correspond to an integer multiple of the
servo spoke timing sensed by the read write channel 54. The length
of the time increment k may be defined based on the rate of change
of head fly height that the fly height estimator 300 is to track
with its fly height estimates. Accordingly, the time increment k
may be decreased (i.e., higher frequency of estimates) to, for
example, track faster head temperature variations.
[0042] Accordingly, fly height estimator 300 utilizes the fly
height table 310 to dynamically estimate head fly height in
response to present air temperature, level of heating by the heater
element 68 (e.g., heater signal 306), length(s) of written data
segment, and/or length of one or more upcoming writes. The fly
height estimator 300 uses the fly height estimates to regulate
heating by the heater controller 304 in an attempt to maintain the
fly height of a selected head within an acceptable range. The fly
height estimator 300 may also selectively inhibit writing by the
write channel 54, via a write gate signal 312, when the estimated
fly height is outside the acceptable range.
[0043] The heater controller 304 can include a heater value table
314 (or other controller logic or controller algorithm) that
translates the fly height estimates from the fly height estimator
300 into digital heater values 306. The digital heater values 306
are converted by a digital-to-analog converter (DAC) 330 into an
analog voltage that is amplified by an amplifier 332 to generate a
heater signal 59 that may be selectively conducted to one of the
fly height adjust (FHA) heater elements 68a-d for a selected one of
the heads 20a-d.
[0044] In FIG. 4, the fly height of a selected head depends upon
the cumulative effects at summing node 340a of head heating by the
write current 342 and the heat output 344 of the heater element.
The fly height of the selected head also depends upon parameters
which are not controlled by the fly height estimator 300, including
environmental conditions 344, such as air temperature, the
configuration of the slider portion of the head 20 which generates
the air bearing 346, and the previous state of head fly height
(indicated as FH.sub.N-1).
[0045] FIG. 5 is a graph that illustrates a typical example of
methods and operation of the fly height estimator 300 and the
heater controller 304 of the fly height controller 57 of FIGS. 3
and 4 for regulating heater element power to obtain and maintain
head fly height within an acceptable range. Referring to FIGS. 4
and 5, the fly height estimator 300 may regulate head heating by
the heater element 68, i.e., regulate heater element power
(V.sup.2), based on a comparison of the estimated head fly height
to various defined control threshold levels. When the estimated fly
height is above a defined write gate upper limit, the fly height
estimator 300 asserts the write gate 312 to prevent data from being
written via the write channel 54 through the head 20. When the
estimated fly height is above an upper target threshold (e.g., an
upper bound of the fly height range of the head 20 while
writing/reading), the fly height estimator 300 commands the heater
controller 304 to generate an upper heater power level V.sup.2
("V.sup.2.sub.Upper Target Threshold") through the heater element
68, which is calibrated to decrease head fly height to below the
upper target threshold.
[0046] When the estimated head fly height decreases below the write
gate upper limit, the fly height estimator 300 de-asserts the write
gate 312 to allow data to be written through the write channel 54
and the head 20. As explained above, the head 20 is heated as data
is written through it, and its fly height can decrease as the
pole-tip-protrusion therefrom increases. While the estimated head
fly height is between the upper and lower target thresholds, the
fly height estimator 300 commands the fly height controller 304 to
generate a steady state level ("V.sup.2.sub.Steady State") of
heater power. The steady state level ("V.sup.2.sub.Steady State")
of heater power is calibrated to be a level that should maintain
head fly height between the upper and lower target thresholds. The
steady state level ("V.sup.2.sub.Steady State38 ) is less than the
upper heater power level V.sup.2 ("V.sup.2.sub.Upper Target
Threshold38 ).
[0047] When the estimated head fly height falls below the lower
target threshold, the fly height estimator 300 commands the fly
height controller 304 to further reduce the heater power level
V.sup.2 to a lower heater power level V.sup.2 ("V.sup.2.sub.Lower
Target Threshold") through the heater element 68. The lower heater
power level V2 ("V.sup.2.sub.Lower Target Threshold") is calibrated
so as to attempt to cause the head 20 to sufficiently cool so that
fly height increases and returns to between the upper and lower
target thresholds. The lower heater power level V.sup.2
("V.sup.2.sub.Lower Target Threshold") is less than the steady
state level ("V.sup.2.sub.Steady State").
[0048] When the estimated head fly height falls below the write
gate lower limit, the fly height estimator 300 asserts the write
gate 312 to prevent data from being written via the write channel
54 through the head 20. With writing inhibited, the head 20 may
sufficiently cool so that its estimated fly height increases
sufficiently above the write gate lower limit so that the write
gate can be de-asserted and writing may resume.
[0049] Under a normal working condition, the upper target
threshold, the lower target threshold, the V.sup.2.sub.Lower Target
Threshold, V.sup.2.sub.Upper Target Threshold, and the
V.sup.2.sub.Steady State are arranged so that the fly height stays
within the range defined between the write gate upper limit and the
write gate lower limit. By properly selecting these values, the fly
height actuator disables reading/writing only during the initial
selection of the target head and changes to the fly height target,
which may reduce/minimize the possible impact on disk drive
performance by the fly height estimator/controller
architecture.
[0050] FIG. 6 shows two graphs that illustrate further methods and
operation of the fly height estimator 300 and heater controller 304
of the fly height controller 57 of FIGS. 3 and 4 for regulating
heater element power to obtain and maintain head fly height
clearance within an acceptable range. Referring to FIG. 6, when the
head 20 is selected, it initially has a fly height that is above
the write gate upper limit and, consequently, the fly height
estimator 300 asserts the write gate 312 to prevent data from being
written through the selected head 20 while the head is flying too
high.
[0051] To prepare for writing, the fly height estimator 300 causes
the heater controller 304 to supply the upper heater power level
V.sup.2 ("V.sup.2.sub.Upper Target Threshold") to rapidly heat the
head 20 and cause its fly height to rapidly drop below the write
gate upper limit. In response to the estimated fly height falling
below the write gate upper limit at time t2, the fly height
estimator 300 de-asserts the write gate 312 thereby allowing
writing to begin. By rapidly heating the head 20 when the head fly
height is above the write gate upper limit, the head fly height may
be rapidly driven below the write gate upper limit and, thereby,
allow writing to more quickly begin following selection of the head
20.
[0052] Although the exemplary flight control illustrated in FIG. 6
has been described in the context of writing, it is not limited
thereto, as it may additionally or alternatively be carried out to
control fly height while reading data.
[0053] FIG. 7 shows two graphs that illustrate further methods and
operation of the fly height estimator 300 and heater controller 304
of the fly height controller 57 of FIGS. 3 and 4 for regulating
heater element power to obtain and maintain head fly height
clearance within an acceptable range. FIG. 7 differs from FIG. 6 in
that a sequence of data writes are carried out with gaps
therebetween and which sufficiently heat the head 20 to cause its
fly height to repeatedly drop below the lower target threshold.
[0054] Referring to FIG. 7, when the head 20 is selected for
writing at time t1, the head 20 initially has an estimated fly
height that is above the write gate upper limit and, consequently,
the fly height estimator 300 asserts the write gate 312 and causes
the heater controller 304 to supply the upper heater power level
V.sup.2 ("V.sup.2.sub.Upper Target Threshold") to cause the head
fly height to rapidly drop below the write gate upper limit. 10055
In response to the estimated fly height falling below the write
gate upper limit at time t2, the fly height estimator 300
de-asserts the write gate 312 thereby allowing writing to begin. As
data is written, the head temperature rapidly increases and the fly
height may correspondingly decreases below the upper target
threshold at time t3, and, in response, the fly height estimator
300 causes the heater controller 304 to reduce the heater element
power to the steady state level ("V.sup.2.sub.Steady State"). With
the steady state level of heater element power, the head fly height
fluctuates as data segments are intermittently written with gaps
therebetween
[0055] The estimated head fly height falls below the lower target
threshold at time t4. In response to the estimated fly height
becoming below the lower target threshold, the fly height estimator
300 causes the heater controller 304 to reduce the heater element
power to the lower heater power level V.sup.2 ("V.sup.2.sub.Lower
Target Threshold"). The fly height estimator 300 causes the heater
controller 304 to increase the heater element power to the steady
state level ("V.sup.2.sub.Steady State") at time t5.
[0056] The estimated head fly height again falls below the lower
target threshold at time t6. In response to the estimated fly
height becoming below the lower target threshold, the fly height
estimator 300 causes the heater controller 304 to reduce the heater
element power to the lower heater power level V.sup.2
("V.sup.2.sub.Lower Target Threshold"). The fly height estimator
300 causes the heater controller 304 to increase the heater element
power to the steady state level ("V.sup.2.sub.Steady State") at
time t7.
[0057] Although not shown, if the head fly height had fallen below
the write gate lower limit, the fly height estimator 300 would have
asserted the write gate 312 to prevent data from being written
through the head 20, which may allow the head 20 to sufficiently
cool so that its fly height may rise above the write gate lower
limit where writing may resume.
[0058] The write gate lower limit feature protects the head disk
interface when, for example, the fly height actuation back-off
capability is not sufficient to compensate for a long write event
and/or when writing pole-tip protrusion occurs faster than can be
compensated for by the fly height actuation. Although some
embodiments have been described in the context of
enabling/disabling write gate using estimated fly during writes,
the invention is not limited thereto. Indeed, in accordance with
various other embodiments, reading can be enabled and disabled in
response to comparison of present fly height with a read gate
window(s) defined between various threshold values. Moreover,
controlling a read gate may be simpler than controlling a write
gate because the fly height estimation while reading does not use
an estimate of write induced pole tip protrusion.
[0059] Although some embodiments the fly height controller 57 have
been described in the context of controlling head fly height in
response to comparisons of estimated fly height to a plurality of
discrete threshold values (e.g., upper target threshold and lower
target threshold) and regulating heater element power between a
plurality of discrete levels (e.g., V.sup.2.sub.Upper Target
Threshold, V.sup.2.sub.Steady State, and V.sup.2.sub.Lower Target
Threshold), the fly height controller 57 is not limited thereto.
The fly height controller 57 may instead regulate heater element
power across any number of intermediate values, within a range of
heater power levels, in response to the estimated fly heights.
Thus, for example, the fly height controller 57 may more
continuously regulate the heater element power between zero and,
for example, the upper heater power level V.sup.2
("V.sup.2.sub.Upper Target Threshold") such that the step-wise
changes illustrated in FIGS. 6 and 7 are replaced by a plurality of
smaller steps and/or continuous transitions, such as along a
polynomial curve, within the defined range.
[0060] In the drawings and specification, there have been disclosed
typical preferred embodiments and, although specific terms are
employed, they are used in a generic and descriptive sense only and
not for purposes of limitation, the scope being set forth in the
following claims.
* * * * *